Q: While driving this afternoon, my temperature gauge shot immediately to full hot at the top of the red range. I pulled over as soon as I could and checked for leaks, and there were none. The engine didn’t seem to be any hotter than normal. I assumed it might be an incorrect gauge and drove straight home. After being parked for about two hours I checked the temperature again, and it was still hot. What do you think is wrong?
— Tara Butler
A: You did a great job explaining how this all unfolded. It sounds like the temperature gauge circuit has encountered a fault that caused it to read at its limit, regardless of actual engine temperature. You were wise to take the reading at face value initially, because an overheating engine can lead to incredibly expensive repair costs. Because the gauge needle moved immediately to hot and continued to stay there with a cooled-down engine, we can suspect it’s likely the only problem — but it’s best to play it safe.
Verifying the actual operating temperature of an engine can be done in several ways. The most accurate is to connect an inexpensive OBD-II generic scan tool, which costs $50-$100 at most auto parts stores and online, to the vehicle’s data link connector, typically found under the instrument panel on the driver’s side. The scan tool’s data list will include engine temperature, obtained by the engine management system’s highly accurate temperature sensor.
Another method is to touch and firmly hold a digital cooking thermometer to the thermostat housing of the engine — generally where the upper radiator hose connects to the engine. An infrared noncontact temperature gun is another, very convenient way to check engine, brake and other automotive system temperatures; point the gun at the thermostat housing. Normal engine operating temperature is between 190 and 200 degrees Fahrenheit, and most electric cooling fans engage at around 220-235 degrees.
Did your “check engine" light illuminate soon after the temperature gauge irregularity? If the vehicle uses a shared engine coolant temperature, or ECT, sensor, this would be likely, as an extremely low or high reading can affect emissions compliance.
Without knowing the vehicle make and model, it’s difficult to say where the fault may lie. You may have a faulty ECT sensor, circuit fault, or instrument panel malfunction. I’d recommend checking the coolant level and gauge operation one more time before driving again. If the gauge continues to read high temperatures with the engine stone-cold, it’s clearly wrong.